Extended Forecast Discussion NWS Weather Prediction Center College Park MD 231 AM EDT Sun Oct 07 2018 Valid 12Z Wed Oct 10 2018 - 12Z Sun Oct 14 2018 ...Central U.S. Heavy Rainfall Finally Moves Eastward... ...Potential T.C. Fourteen Likely to Affect the Gulf Coast and Portions of the East... ...Moisture from Sergio Should Reach into Arizona/Southern Rockies... ...Pattern Overview and Guidance/Predictability Assessment... Expect an overall pattern transition that will promote a trend from very active weather toward decreasing coverage and intensity of precipitation across the lower 48 states. While an initially strong western Atlantic ridge aloft weakens/recedes, a strengthening system tracking from near the upper MS Valley into eastern Canada will push a cold front into and across the East during Wed-Fri. The front will be accompanied by areas of moderate-heavy rainfall, and is likely to interact eventually with Potential Tropical Cyclone Fourteen which based on the 0300 UTC NHC advisory should be nearing the Florida panhandle by early Wed. At the same time energy will be feeding into the established western U.S. upper trough. As the upstream ridge aloft gradually builds toward the West Coast/western Canada, some of the western energy should proceed eastward while the rest lingers over/near the Southwest. It is possible that some of this latter energy may interact with the upper reflection of Sergio is it reaches the region in a few days. By the end of the period consensus evolution that shows a West Coast/western Canada mean ridge, a modest weakness near the Southwest, and mean troughing settling into the east-central U.S. corresponds well to teleconnection-favored flow relative to negative height anomalies that have been forecast just south of the Aleutians/Alaska Peninsula late in the period. As for forecast specifics, guidance has been displaying some day-to-day timing oscillation for the system tracking from the upper Midwest/Great Lakes into Canada. Latest array of solutions is a little faster than yesterday after having trended slower versus the previous day. Farther southward model/ensemble spread is still very broad for Potential Tropical Cyclone Fourteen. Recent CMC/GFS runs have represented the faster side of the spread but the 00Z GFS has adjusted closer to the official track from the 0300 UTC advisory. On the other hand ECMWF/ECMWF ensemble runs have been extremely slow and possibly exhibiting biases that existed for some previous tropical systems during at least a portion of their existence. By late week the 12Z ECMWF/ECMWF mean position may not be too far off from a possible frontal wave though. Broadly speaking guidance seems to be trending toward an intermediate scenario over/near the Southwest. The ECMWF/ECMWF mean have toned down their expected troughing over the past day while the GFS has made a notable adjustment toward having more lingering energy instead of quickly raising heights aloft as in previous runs. There is still considerable ensemble spread and the scale of energy is small enough to suggest predictability in the lower half of the spectrum. An additional unknown is the degree to which this energy may interact with the upper reflection of Sergio as it arrives late in the week. Latest models/ensembles are starting to become a little more similar for the handling of North Pacific energy that should enter western North America by late week and then feed into the mean trough likely reaching the central/east-central states next weekend. This energy would support a clipper type system that tracks near the upper Great Lakes by early day 7 Sun. The updated forecast started with an operational model blend on day 3 Wed followed by gradually increasing weight of 18Z GEFS/12Z ECMWF means so that day 7 Sun had 70 percent total means with the 18Z GFS/12Z ECMWF comprising the rest of the blend. The initial blend was adjusted to account for tropical system forecasts. ...Weather Highlights/Threats... The vigorous storm system tracking northeastward from the upper MS Valley will anchor a cold front that will be accompanied by areas of moderate-heavy rainfall as it crosses the eastern U.S. Meanwhile Potential Tropical Cyclone Fourteen is forecast to be near the Florida panhandle early Wed and then continue northeastward as the front approaches. This system will provide additional moisture input with associated rainfall most likely to occur over parts of the Southeast/Mid-Atlantic. However note that there is still significant uncertainty for exact track/timing of Potential T.C. Fourteen and thus for precise location/intensity of rainfall. Expect drier conditions to move into the East late week into the weekend. In the cold sector, parts of the extreme northern Plains may see a period of snow early in the period. The Great Basin and northern-central Rockies should trend drier after a period of snow and lower elevation rain that will cross the region in the Wed-Thu time frame. Farther southward there are still some question marks for timing but a reasonable consensus exists toward Sergio's moisture primarily affecting the southeast half of Arizona and parts of the southern Rockies. The axis of heaviest rainfall would be somewhat southeast of locations that received the most rain from Rosa. Some of the moisture from Sergio should extend into the Plains but thus far there has been poor guidance agreement/continuity for location and intensity of rainfall. Much below normal temperatures covering areas from the Interior West into northern-central Plains Wed-Thu should gradually moderate and push slowly southeastward with time. Some minus 20-30F anomalies for highs are possible early in the period over the northern half of the High Plains and vicinity. Also early in the period the eastern U.S. will be extremely warm with min temperatures particularly anomalous (20-25F above normal at some locations). Numerous locations may see daily records for warm mins. Cold frontal passage will bring eastern U.S. temperatures down close to or even a little below normal by next weekend. Expect the West Coast states to see slightly above normal readings by next weekend as upper ridging approaches. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml